This invention relates to the production of silica by hydrolysis of a silicate ester under controlled conditions.
It is well known to produce silica by a simple procedure wherein a sodium silicate solution is precipitated with an acidic material. This technique is inexpensive and gives a silica which inherently has sufficient strength to resist severe shrinkage of the pores during simple drying techniques such as oven drying, tray drying, spray drying, or drying under a heat lamp. Such a silica has relatively small pores, however, which is a disadvantage in some applications. Nevertheless, it can be used as a support for a chromium catalyst for olefin polymerization in a solution process with excellent results. However, in a slurry system such a catalyst tends to produce polymer having an excessively high molecular weight because the temperature, which is simply raised in solution systems to decrease molecular weight, must be kept low enough that the polymer does not go into solution.
It is known that titanium affects the polymerization activity of silica supported chromium catalysts in a way that is of special importance in slurry polymerizations. In particular, it gives a support which results in lower molecular weight polymer. In order to take full advantage of the improvement which can be imparted to the melt index capability through the use of titanium in accordance with the prior art, the titanium had to be coprecipitated with the silica and the resulting hydrogel (cogel) dried by a more expensive azeotrope distillation or washing with a liquid oxygen-containing water soluble organic compound so as to remove the rather substantial amount of water present because of the formation of the hydrogel in an aqueous system.
Also, in any procedure involving the precipitation of silica from a sodium silicate, the sodium ions must be washed from the final product which is a serious disadvantage.
It is known in the art to prepare a silica by the hydrolysis of an ester but this has heretofore resulted in the production of hard beads of silica which are not ideally suited for applications such as supports for chromium catalysts for olefin polymerization.